fundamental aerosol studies with an ultrasonic nebulizer

Fundamental Aerosol Studies with an Ultrasonic Nebulizer

M A T T H E W A. TARR, G U A N G X U A N Z H U , and R I C H A R D F. B R O W N E R * School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, Georgia 30332-0400

Laser scattering particle size measurements, together with aerosol and vapor mass transport, were used to characterize the aerosol generated with an ultrasonic nebulizer (USN). Several fundamental aspects of the ultrasonic primary and tertiary aerosols were studied, including drop size distributions, Sauter mean diameters, analyte mass transport, and solvent mass transport. These characteristics were compared with comparable data obtained with a concentric, all-glass (Meinhard type) pneumatic nebulizer. It was found that the ultrasonic nebulizer generated a primary aerosol which had a substantially broader size distribution than the pneumatic nebulizer. However, overall both analyte and solvent mass transport for the USN were much greater than for the pneumatic nebulizer.

Index Headings: Ultrasonic nebulizer; Particle size distribution; Laser scattering; Atomic emission spectrometry.

INTRODUCTION

One of the major barriers to achieving improved detection limits in inductively coupled plasma atomic emission spectroscopy (ICP-AES) arises from limitations in- herent in traditional sample introduction techniques. A typical combination of a pneumatic nebulizer (PN) and spray chamber used for ICP-AES results in analyte transport efficiencies to the plasma in the range of 1 to 2 %. Ultrasonic nebulizers (USN) are generally assumed to produce a uniform-sized aerosol with a small mean diameter, and so have been used on a number of occasions in place of pneumatic nebulizers in an attempt to increase the analyte mass transport to the plasma, and hence to lower detection limits. 1-s However, the precise relationship between the rate of delivery of analyte to the plasma and the detection limit achieved is a complex one which involves many additional factors. These in- clude: (1) the particle size distribution of the aerosol reaching the plasma, (2) the condensed phase solvent loading, and (3) the vapor solvent loading. Analytical detection limits are usually defined in terms of both signal magnitude and background noise, and both of these parameters are determined directly by the transport properties of the analyte and solvent aerosols and also by the transport properties of solvent vapor.

In describing the overall analyte transport process, the size distribution of the primary aerosol (e.g., the aerosol formed at the nebulizer) as well as that of the tertiary aerosol (e.g., the aerosol leaving the spray chamber) must be known. There are very few reports in the literature which describe any fundamental properties of aerosols generated by USN, and those reports all describe the properties of tert iary aerosols, and so give no information on the actual operation of the ultrasonic nebulizer it- selfY ,9,1° Additionally, it is of interest to determine wheth-

Received 24 April 1991; revision received 30 May 1991. * Author to whom correspondence should be sent.

er the drop sizes of aerosols generated by ultrasonic nebulizers of the type used in ICP-AES may be described with the use of relationships originally developed for nebulizers of a different type.

This study presents for the first time data on the important aerosol characteristics described above. Laser Fraunhofer scattering is used to determine particle size distributions of both primary and tertiary aerosols generated ultrasonically, and also of pneumatically generated aerosols for comparison purposes. Aerosol collection techniques are used to determine analyte and solvent mass transport. The results of these studies form the basis for quantitative explanation of the improved detection limits which are often observed with USN. Ad- ditionally, the influence of operating parameters on the observed aerosol properties is described.

EXPERIMENTAL

I n s t r u m e n t a t i o n . The ultrasonic nebulizer and its spray chamber were fabricated in house, on the basis of the designs of Fassel and Bear. 1 The transducer incorporated in this design was the model CPMT obtained from Chan- nel Products, Inc. (Chesterland, OH). The nebulizer and spray chamber design is illustrated in Fig. 1. The sample delivery tube was made by cutting one end of straight 0.01-in.-i.d. HPLC tubing at a 45 ° angle. This end was made parallel with the quartz plate of the nebulizer. The tube was then mounted to the nebulizer via a horizontal translator. The tube could therefore be positioned very precisely with respect to the quartz plate. The separation between the solvent delivery tube and the nebulizer was set at values of 0.1, 0.2, 0.5, and 1.0 mm. The nebulizer was powered by a Plasma-Therm (Vorhees, NJ) UNPS-1 generator, operating nominally at 1.35 MHz. The generator was tuned to the resonant frequency of the transducer by maximizing incident power while minimizing reflected power. Incident powers of 20, 25, 30, 35, and 40 W were used. All experiments used the same transducer, except where otherwise noted.

Solvent was introduced to the nebulizer with the use of one of two HPLC pumps. For pure water, a Consta- metric Model III pump was used. For solvents containing nitric acid, a Waters M-6000A pump was used. Flow rates of 0.5, 1.0, 1.5, and 2.0 mL/min were used. In operation, liquid from the sample delivery tube runs down the face of the transducer in a stream and is nebulized by inter- action with the oscillations of the crystal.

Particle size measurements were carried out with a Malvern (Southborough, MA) 2600c Droplet and Par- ticle Sizer. This instrument relies on laser Fraunhofer scattering to determine the particle size distribution. A helium/neon laser beam (9 mm diameter) is passed through the aerosol of interest. Light scattered by the

1424 Volume 45, Number 9, 1991 0003-7028/91/4509-142452.00/0 © 1991 Society for Applied Spectroscopy

APPLIED SPECTROSCOPY

aerosol is collected on a series of concentric, semi-circular detectors. The light pattern on the detector array is mathematically transformed to give a particle size distribution of the aerosol. The technique is based on droplet volume, and results in a volume distribution. When the primary aerosol (no spray chamber) was being measured, the laser light was attenuated with a neutral-density filter (0.15 O.D.) to avoid saturation of the detector caused by high aerosol density. Furthermore, a baffle was used to shorten the laser pathlength through the aerosol, keep- ing the scattered light intensity below saturation. This baffle consisted of a piece of aluminum foil with a 5-mm slit placed vertically in front of the center of the transducer so that the laser sampled only the aerosol produced near the central vertical axis of the transducer. This design presumably did not discriminate against any par- ticular droplet size. The primary aerosol spanned about 20 mm in height, with only the central 9-mm portion being sampled by the laser.

Measurements of the tertiary aerosol were made directly after the aerosol left the spray chamber, with argon at 1 L/min as carrier gas. Except for slight fluctuations in the aerosol path, the laser beam sampled the entire tertiary aerosol. In all particle size experiments, the liquid flow rate was held constant while the power was varied; then the flow rate was adjusted to the next value and again held constant while the power was varied. Each measurement acquired was a time average of 500 scans, requiring a total measurement time of approximately twenty seconds. Each value reported is an average of three consecutive measurements.

In order to determine the short-term reproducibility of the tertiary aerosol properties, a 30-W 1-mL/min water aerosol was measured after it left the spray chamber, over a ten-minute time period. Aerosol was measured for five seconds (100 scans) every thirty seconds.

All reported diameters are based on particle size data in the range 1.22 to 118 #m. Although the Malvern instrument is capable of extrapolating values below 1.22 /~m, these data were considered to be unreliable for the purpose of these experiments. A small amount of aerosol was experimentally observed below 1.6 #m (< 1% by volume); however, these data were eliminated for the cal- culation of number distributions. This was done to min- imize propagation of errors in these calculations at small diameters.

Transport Measurements. Solvent transport measurement was accomplished in the following manner: the nebulizer was set up with the spray chamber attached. Argon at a flow rate of 1.0 L/min was used to sweep the aerosol out of the spray chamber. The solvent delivery tube was set at 0.5 mm away from the nebulizer quartz plate. Liquid flow rates and power settings were varied as stated above, with the use of 0.5% v/v nitric acid (aqueous) as solvent. The aerosol was drawn through a glass U-tube packed with 6-16 mesh silica gel desiccant (Fisher). The weights of the U-tube before and after aerosol collection were used to determine the solvent transport.

To determine the analyte transport, we used a similar method. In this case, a 1000-ppm Mn solution in 0.5% nitric acid (aqueous) was nebulized. Gelman glass fiber filters were used to collect the aerosol. The filter was

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Fro. 1. Schematic of ultrasonic nebulizer. A, solvent delivery tube; B, Ar carrier gas inlet; C, glass spray chamber; D, drain; E, cooling water inlet/outlet; F, rf power in; G, Teflon ® mount; H, horizontal translator; J , piezoelectric transducer; K, quartz coupling plate; L, electrical leads. P and T indicate where the primary and tertiary aerosols were measured, respectively. Scale is approximate.

then extracted with nitric acid. The extract was then analyzed for Mn by ICP-AES to determine the analyte transport. A blank aerosol containing no Mn was also collected in this manner, and the residual Mn signal from this blank was used for background correction.

As with the particle size measurements, liquid flow rate was held constant at each of four values while power was varied. At each flow rate, collections were done in duplicate at the 30-W power level in order to record the experimental error.

Reagents. Filtered deionized water, Fisher reagent ACS nitric acid, and Fisher certified ACS manganous sulfate monohydrate were used in these studies.

PRESENT THEORY OF ULTRASONIC NEBULIZATION

Aerosol formation by ultrasonic nebulizers is a result of the transfer of acoustical energy to the nebulized liquid. A piezoelectric transducer is made to oscillate at its characteristic frequency, which is dependent upon the transducer's material and geometric dimensions. Liquid is run across the face of the transducer or a coupling material attached to the transducer. Within the liquid, longitudinal waves are formed by particle motion along the direction of the axis of propagation. 11 In addition, capillary surface waves are formed on the surface of the liquid.ll The wavelength, X, of the capillary waves is given by Kelvin's formula, TM

X = ( 8 7 r d l p r 2 ) '/a (1)

where a is the surface tension, p is the density of the liquid, and v is the transducer frequency. Lang ~2 further reports that the number median particle diameter, Dn, is given by

D, = 0.34X. (2)

According to Eqs. 1 and 2, D, is dependent only on the liquid properties and the frequency of excitation. These equations show no dependence of the number median particle diameter on nebulizer power, nor do they give any indication of the width of the particle size distri-

APPLIED SPECTROSCOPY 1425

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bution. Lang 12 does experimentally illustrate the relationship between frequency and D,; however, Browner 13 indicates that, above 800 kHz, D, may no longer be dependent on frequency, but rather it may become dependent upon the power density on the liquid surface. 14 Such changes in aerosol properties indicate a possible change in the mechanism of aerosol formation at low and high frequencies.

There are two general theories as to the mechanism of aerosol formation: cavitation and geyser formation. ~3 Fig- ure 2 illustrates these two processes. Both Lang 12 and Basset and Bright 15 have observed capillary surface waves of the appropriate frequency by photographic methods. Although both mechanisms are cited in the literature, no sound evidence for the predominance of either mechanism is available.

From the perspective of atomic spectrometry, the number median diameter is not a very useful parameter. Of much greater importance is the mass median diameter, which can be approximated by the Sauter mean diameter, ds. The Sauter mean diameter is expressed as

d~ = Z ( d 3 A n ) / Z ( d 2 ~ n ) (3)

where An represents the number of drops of diameter d. The Sauter mean diameter represents a mean based on

T A B L E I. Number median diameter, volume median diameter, and Sauter mean diameter for various flow rates and nebulizer powers. Pri- ma ry aerosols of 100% water.

N u m b e r Volume Sau te r m e d i a n med ian m e a n

Flow rate Power d iamete r d iamete r d iamete r (mL/min ) (W) (#m) (#m) (#m)

0.5 20 4.5 5.5 3.7 25 4.7 10.0 5.8 30 4.9 10.0 5.6 35 4.7 9.0 5.0 40 4.6 10.0 5.4

1.0 20 4.4 5.1 3.6 25 4.5 8.5 5.O 30 4.9 13.0 6.3 35 4.9 11.0 5.4 40 4.8 10.4 5.2

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2.0 20 5.0 8.1 4.9 25 4.9 10.8 6.0 30 5.0 13.2 6.2 35 5.0 14.2 6.3 40 4.9 14.1 6.1

the volume-to-surface area ratio. In ICP techniques, it is important to rapidly and efficiently convert droplets to atomic and ionic species. Droplets with small Sauter mean diameters will possess a large ratio of surface area to unit mass, making the conversion to atomic and ionic species easier. Since the Sauter mean diameter is a much more important factor in atomic spectrometry than the number median diameter, it would be useful to have a theory written in terms of this factor. The relationships which exist between the number median diameter, the Sauter mean diameter, and the experimental data obtained in this study will be discussed below.

RESULTS AND DISCUSSION

Comparison of Theory with Experimental Data. The data produced by the Malvern particle sizer are particle size distributions as a function of volume percent of aerosol, whereas the earlier theoretical discussion relates transducer frequency to the aerosol number median diameter. In order to compare experimental results with theoretical predictions, the experimental data must be transformed to a numerical count distribution. Such transformations can be easily performed by converting diameter to volume. To transform from a volume distribution to a number distribution, one divides the total volume at each diameter by the corresponding volume of each particle at that diameter to give the total number of particles at each diameter. Mathematically, the trans- formation is as follows:

Ni = V o / V i (4) N , = 6 Vobrd, 3 (5)

where N~ is the number of particles at diameter di (cm), V~ is the volume (cm 3) of each particle at diameter di, and V0 is the total volume (cm 3) of all particles of diameter d~. In this treatment, all particles are assumed to

1426 Volume 45, Number 9, 1991

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10 100 200 Diameter (pm)

Fro. 3. Ul t rasonic p r imary aerosol vo lume (A) and n u m b e r (B) dis- t r ibu t ions for water.

be spherical. In performing such mathematical transformations, one must keep in mind that the instrumental measurement is based on volume, and may not transform to the exact number distribution. One must also keep in mind that any experimental error at the small volume diameters will result in large errors in the experimentally derived number diameters. Furthermore, since the current theory (Eq. 2) gives only the number median di-

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Table I lists the Sauter mean diameter, the volume median diameter, and the experimentally derived number median diameter for primary aerosols of 100% water. The values for the experimentally derived number median diameters should be taken as estimates only. With the application of Eqs. 1 and 2, with cr = 73.05 dynes- cm -1, p = 1 g'cm -3, and ~ = 1.35 MHz (parameters for water and the transducer used), calculations yield the theoretical values of ~ = 1.00 x 10 -3 cm and D, = 3.41 ttm. This value is fairly close to the average experimental number median diameter of about 5 #m. Figure 3 illustrates an experimentally derived number distribution and the volume distribution from which it was derived.

Several important observations may be made on the basis of these data. First, the theoretical values and the experimental values agree reasonably well. Second, the experimentally derived number med ian diameters show almost no variation with changes in power and flow rate. This fact is clearly not true for the Sauter mean diameters, as will be discussed below. It is important to note that, in deriving the number distribution from the volume distribution, the small particles are heavily weight- ed, while the large particles are of much less importance. This fact may account for the lack of variation of D, with power and flow rate, while ds does show some variation with changes in these parameters. As previously stated,

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APPLIED S P E C T R O S C O P Y 1427

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the volume distribution is much more important to ICP applications than is the number distribution.

Although the number median diameter data are in agreement with the current theory, the utility of the theory in atomic spectrometry is rather limited. The current theory gives little information of importance in this field since it contains no information about the aerosol volume distribution. Though the current study does not present a new theory, the relationship between the experimental parameters and the important aerosol properties is shown. Such information is important in un- derstanding the fundamental processes occurring in ultrasonic nebulization. Further study will be necessary in order to elucidate a new, more useful theory.

Primary Aerosol Particle Size Distributions. Particle size data were obtained as a function of transducer power and liquid flow rate. Particle size distributions for the primary aerosol of 100% water are given in Fig. 4. Clearly the particle size distributions are very polydisperse and become more so with increasing power. Under most conditions, it was observed that 90% (by volume) of the aerosol had diameters greater than about 4 ~m. At the same time, 90 % of the aerosol had diameters less than about 30 ~m. From these data we can conclude that significant amounts of the aerosol are distributed over at least an order of magnitude in drop sizes.

Figure 5 illustrates the effects of varying power and flow rate on number median diameters and Sauter mean diameters for the primary aerosol. As discussed earlier, the number median diameters are relatively constant with power and flow rate, while the Sauter mean diameters show substantial variation with these parameters. This result correlates with the observation that the major changes in aerosol distribution occur at the larger drop sizes, and that the relatively small number of large drops contribute very little to the number distribution.

At all flow rates, values of the Sauter mean diameter increase when nebulizer power exceeds 20 W. At a power setting of 20 W, most of the liquid flows off of the nebulizer without being nebulized, as apparently too little power is available to nebulize the water effectively. At a

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FIG. 6. Comparison of primary pneumatic (A) and primary ultrasonic (B) volume distributions.

setting of 25 W, a much greater amount of aerosol is nebulized, and an abrupt change in Sauter mean diameter also occurs, which indicates a possible change in the mechanism of nebulization. In general, values of the Sau- ter mean diameter increase significantly with increasing power up to a maximum, and then decrease only slightly as the power is further increased. Prior to these observations, it was anticipated that operation of a USN at higher power would lead to the formation of smaller drops as a consequence of the greater oscillation energy available for nebulization. Unlike pneumatic nebulizers, USNs do not appear to form smaller droplets as the energy input to the nebulizer is increased.

For comparison, Fig. 6 shows the primary aerosol distribution of 100% water for both a pneumatic (Meinhard concentric nebulizer) and an ultrasonic nebulizer. In this figure, the multimodal distribution of the ultrasonic aerosol is apparent. Furthermore, it appears that the primary pneumatic aerosol is more monodisperse than the primary ultrasonic aerosol. This result is contrary to the common belief that ultrasonic nebulizers produce narrow aerosol distributions. The y-axis in the figure is reported in units of volume percent, which are relative values for the aerosol within each plot. Therefore, the height of each peak is a measure of the aerosol monodis- persity, and not a measure of the absolute aerosol density. Although the ultrasonic aerosol distribution is broad, the ultrasonic nebulizer produces an aerosol of much greater particle density than the pneumatic nebulizer. When the aerosol is passed through a spray chamber in ICP analysis, droplets too large to be introduced into the ICP are eliminated. When excessively large droplets have been eliminated in this manner, there is expected to be a direct correlation between the amount of analyte reaching the plasma and the emission signal, assuming no solvent interactions take place. Even though the ultrasonic aerosol has many droplets that are too large to reach the plasma, the high aerosol density ultimately results in the production of many more appropriately sized droplets than found with the pneumatic nebulizer.

Tertiary Aerosol Particle Size Distributions. Tertiary aerosols of both 100 % water and 0.5 % nitric acid (aqueous) were measured at the spray chamber exit. Although


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FIG. 7. Tertiary ultrasonic aerosol distributions for 1.0 mL/min water: (a) 20, (b) 30, and (c) 40 W; and 1.0 mL/min 0.5% HNO3 (aq): (d) 20, (e) 30, and (f) 40 W. Sauter mean diameters are indicated.

the aerosol at this point is generally too heavily laden with solvent to give useful signals from the ICP, the aerosol was measured here to gain fundamental knowl- edge of the aerosol before desolvation. In actual use with an ICP, the ultrasonic nebulizer is typically connected to a desolvation system (see Fassel and Bear1). Further- more, use of the spray chamber results in a compound system. The aerosol properties are no longer a function of the nebulizer alone, but are now a function of the entire nebulizer/spray chamber system.

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FIG. 8. Three-dimensional plot of Sauter mean diameter as a function of liquid flow rate and nebulizer power for an ultrasonic tertiary aerosol of water.

Figure 7 compares the particle size distributions of the two nebulized liquids. Use of the nitric acid solution surprisingly results in an aerosol with significantly smaller particle sizes than those seen with pure water. In comparison with Fig. 4 it may also be seen that the spray chamber is effective in eliminating drops greater than about 20 #m for 100% water.

The behavior of the nebulizer as a function of both power and liquid flow rate is of considerable interest. In this context, three parameters are of major importance: the Sauter mean diameter, the % volume concentration of the aerosol (% of measurement volume consisting of droplets), and the span (a measure of the width of the aerosol distribution). For 100% water, a three-dimensional plot of Sauter mean diameter vs. power and flow rate is given in Fig. 8. Figure 9 illustrates the same plot for % volume concentration as a function of power and flow rate.

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APPLIED S P E C T R O S C O P Y 1429

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FIG. 10. Sauter mean diameter (a) and % volume concentration (b) versus nebulizer power for 0.5 % HNO~ (aq) tertiary ultrasonic aerosol. Liquid flow rates are: 0.5 (*), 1.0 (O), 1.5 (D), and 2.0 (z~) mL/min.

A high % volume concentration is desired as this will result in a high mass transport to the plasma. From Fig. 9, we see that % volume concentration increases as both power and flow rate increase. The Sauter mean diameter increases with power, but remains almost constant with changing flow rate (at constant power). In so far as op- t imum ICP signal is obtained with smaller particles, the low power settings would seem to be advantageous. How- ever, any gain in the signal-to-noise ratio (SNR) achieved by producing smaller droplets at low power would prob- ably be more than offset by the decrease in analyte transport, since operation at low power results in a much less dense aerosol than does operation at high power. This analysis agrees with the observations of Boumans and De Boer. 1° The span did not change significantly except for a slight increase with increasing power.

Another variable parameter that was examined was the separation between the solvent delivery tube and the quartz plate of the nebulizer. Experiments were run with separations of 0.1, 0.2, 0.5, and 1.0 mm. Although little variation in the particle size distribution was observed over this range, it was noted that nebulizer stability was affected by the separation distance. At close distances

(0.1 and 0.2 mm), the nebulizer was difficult to operate reliably at low flow rates (0.5 and 1.0 mL/min) with high powers (35 and 40 W). All particle size testing with the use of 0.5% nitric acid (aqueous) was therefore carried out with a separation of 0.5 mm.

For the particle size data from the use of 0.5% nitric acid (aqueous), two-dimensional plots of Sauter mean diameter and % volume concentration vs. power at constant flow rate are given in Fig. 10. The aerosol densities increase with increasing power and flow rate, with a slight tailing off at high power. The Sauter mean diameters also increase with increasing power, but decrease with increasing flow rate. Results for a second transducer were very similar, showing the same trends.

As stated earlier, the variations in Sauter mean diameter do not follow the expected trends. Although the spray chamber plays a significant role in determining the tertiary aerosol drop size distribution, it is apparent from the data that varying the operating conditions for the nebulizer does affect the tertiary distribution. For both transducers, higher flow rates yielded smaller drop sizes. Furthermore it seems that the effect of power on the aerosol may vary somewhat from transducer to transducer. These observations are most likely due to changes in the mechanism of drop formation. Surface power density, liquid film thickness, and surface properties of the liquid are undoubtedly important factors involved in controlling the droplet formation process.

It is interesting to note that in Fig. 10 the % volume concentrations for 1.5 and 2.0 mL/min seem to parallel each other. In these cases, it seems likely that the nebulizer has reached a maximum aerosol output for the applied power and, despite increased flow, cannot nebulize more liquid. This trend appears again in the analyte and solvent transport data discussed later.

In experiments of short-term reproducibility, the following results were observed: d, = 4.49 _+ 0.03 and % volume concentration = 0.0052 ± 0.0003. Clearly, the value of the Sauter mean diameter remains very constant with time, but the % volume concentration shows much poorer precision. The change in % volume concentration can be attributed to two factors. First, as the aerosol exits the spray chamber, its boundaries are able to move slightly. This movement may result in a reduction in the amount of aerosol intersected by the laser, effectively reducing the observed % volume concentration. Second, actual fluctuations in the aerosol production may be re- sponsible for the change in aerosol density. Such changes may be crucial in achieving stable ICP-AES signals. Fur- ther work is necessary in order to determine the overall effect of these fluctuations on the ICP-AES signal.

Solvent and Analyte Transport. Among the most important parameters in achieving high SNR are high sample transport and low solvent transport. The former increases the signal, while the latter generally decreases the background. There are two ways of reporting transport. The first is as transport efficiency, e, and the second is mass transport, W. Efficiency is the percent of the originally introduced sample reaching the plasma. Al- though efficiency is the most common way of describing analyte transport, it does not fully characterize the transport, as it is dependent on the flow rate of introduced sample. Mass transport is a much more useful parameter


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0

(a)

20 25 30 35 40 Power (W)

35 ~ .30 o~25

o~20 ~15 ._o 10 U.J 5

0 20 25 30 35 40

Power (W) FIG. 11. Tertiary ultrasonic aerosol analyte mass transport (a) and efficiency (b) for Mn in 0.5% HNO3 (aq). Flow rates are: 0.5 (*), 1.0 (©), 1.5 (D), and 2.0 (A) mL/min. The dashed lines indicate the mass transport and efficiency for a Meinhard pneumatic nebulizer for Mn in 1% HNO3 (aq) (1.0 mL/min) and 0.9 L/rain Ar.

3O

~ '25

~ 2 0

=°15 ~ 10 o _

u.l 5

@

20 25 30 35 40 Power (W)

FIG. 12. Tertiary ultrasonic aerosol solvent mass transport (a) and efficiency (b). Flow rates are: 0.5 (*), 1.0 (©), 1.5 (D), and 2.0 (A) mL/ min 0.5% HN03 (aq). The dashed lines indicate the mass transport and efficiency for a Meinhard pneumatic nebulizer with 1.0 mL/min 1% HNO~ (aq) and 0.9 L/min Ar.

because it is proportional to atomic emission intensities. Mass transport is reported in mass per second. The current study relies on direct methods to measure both analyte and solvent transport.

Figure 11 illustrates the analyte mass transport and transport efficiency vs. power at various flow rates. Figure 12 shows the analogous plots for the solvent. All collections were carried out after the spray chamber and with no desolvation. The analyte mass transport shows increasing transport with increasing power, with a gradual leveling off or decline at higher powers. Mass transport also increases with increasing flow rate. Note, however, that the plots for 1.5 and 2.0 mL/min are essentially the same. This trend was observed for the % volume concentration measurements above, and is presumably due to overloading of the nebulizer. Analyte transport efficiency decreases with increasing flow rate because the divisor (flow rate) becomes large more rapidly than does the amount of analyte transported. Clearly illustrated in the figure is that the mass transport and efficiency measurements show opposite trends with increasing flow rate. The importance of reporting mass transport is thus made obvious. In this case, optimizing analyte transport efficiency would result in degrading ICP-AES signal, whereas optimizing analyte mass transport would enhance ICP- AES signal.

The plots for solvent transport (Fig. 12) show very similar trends in comparison to those for analyte transport. The analyte and solvent transport efficiencies are almost identical. However, the solvent mass transport is roughly 1000 x the analyte mass transport, a reasonable result considering that the test solution was 1 ppt in analyte. In addition, the % volume concentration curve in Fig. 10b and the analyte and solvent transport curves

are all very similar, indicating good agreement between the different experiments.

Compared to results for a pneumatic nebulizer, the maximum observed transport at 1.0 mL/min liquid flow for the USN was roughly nine times greater (3.77 #g/s for USN vs. 0.43 #g/s for PN). Not surprisingly, the solvent transport for the USN was roughly eight times greater than for the PN (3970 #g/s for USN vs. 500gg/s for PN). These facts clearly illustrate the greater aerosol output of the ultrasonic nebulizer.

The analyte transport efficiency data reported here are comparable to results obtained in earlier studies by Olson e t a l . 2 and Olivares and Houk, 16 in which both groups used only one liquid flow rate and a single nebulizer power.

Table II lists the experimental error observed at each flow rate at 30 W power for both analyte and solvent transport.

CONCLUSIONS

This study has demonstrated that ultrasonic nebulizers generate aerosols with significantly broader particle size distributions than the more commonly used concen-

TABLE II. Analyte and solvent mass transport for several liquid flow rates. Nebulizer power, 30 W; Ar carrier flow, 1 L/rain.

Flow rate Analyte mass transport Solvent mass transport (mL/min) (/.tg/s) (#g/s)

0.5 2.4 _+ 0.3 2200 _+ 30 1.0 2.64 _+ 0.01 3030 _+ 60 1.5 3.35 +_ 0.09 3600 +_ 100 2.0 3.0 ± 0.3 3930 ± 30

APPLIED SPECTROSCOPY 1431

tric pneumatic nebulizers. Furthermore, increasing the input of rf energy to an USN does not result in smaller drop sizes, whereas increasing the available kinetic energy to a PN does produce smaller droplets. Variations in liquid flow rate also give interesting results, showing that smaller tertiary drop size distributions result from increasing liquid flow. Such results illustrate the need for further study to understand more precisely the aerosol formation mechanisms which occur with USN.

The current study emphasizes the importance of char- acterizing the parameters of analyte transport, solvent transport, and particle size distribution in atomic spectrometry. Such factors are of great importance when sample introduction systems are being modeled.

ACKNOWLEDGMENTS

The authors acknowledge Ken Williams for his assistance in ma- chining several of the nebulizer parts. We also express thanks to Car- men Germano of Channel Products, Inc. for supplying the piezoelectric transducers used in this study. This work was supported by the Na- tional Science Foundation under Grant No. CHE88-08183.

1. V. A. Fassel and B. R. Bear, Spectrochim. Acta 41B, 1089 (1986). 2. K. W. Olson, W. J. Haas, Jr., and V. A. Fassel, Anal. Chem. 49,

632 (1977). 3. P. D. Goulden and D. H. J. Anthony, Anal. Chem. 56, 2327 (1984). 4. C. E. Taylor and T. L. Floyd, Appl. Spectrosc. 35, 408 (1981). 5. K. E. Lafreniere, G. W. Rice, and V. A. Fassel, Spectrochim. Acta

40B, 1495 (1985). 6. R. J. Thomas and C. Anderau, Atom. Spectrose. 10, 71 (1989). 7. T. J. Johnson, P. Cassagne, and D. Rupp, Am. Lab. 21(5), 112

(1989). 8. G. A. Petrucci and J. C. Van Loon, Spectrochim. Acta 45B, 959

(1990). 9. R.H. Clifford, I. Ishii, and A. Montaser, Anal. Chem. 62, 390 (1990).

10. P. W. J. M. Boumans and F. J. De Boer, Spectrochim. Acta 30B, 309 (1975).

11. J. M. Mermet, C. Trassy, and P. Ripoche, "Design of a New Ul- trasonic Nebulizer for Routine Analysis in ICP-AES," in Devel- opments in Atomic Plasma Spectrochemical Analysis, R. M. Barnes, Ed. (Heyden, London, 1981), p. 245.

12. R. J. Lang, J. Acoustical Soc. Amer. 34, 6 (1962). 13. R. F. Browner, "Fundamental Aspects of Aerosol Generation and

Transport," in Inductivity Coupled Plasma Emission Spectros- copy: Part II, P. W. J. M. Boumans, Ed. (John Wiley & Sons, New York, 1987), Chap. 8, p. 244.

14. R. F. Browner and A. W. Boom, Anal. Chem. 56, 875A (1984). 15. J. D. Basset and A. W. Bright, J. Aerosol Sci. 7, 47 (1976). 16. J. A. Olivares and R. S. Houk, Anal. Chem. 58, 20 (1986).


fundamental aerosol studies with an ultrasonic nebulizer

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